Urea nitrogen is subject to volatilization loss as ammonia (NH3) gas if not properly incorporated shortly after application. Once incorporated into the soil, urea and other ammonium (NH4+)-forming nitrogen fertilizers will eventually be converted to nitrate (NO3-), which is prone to leaching or denitrification.

What Are Nitrogen Fertilizer Additives?

Nitrogen fertilizer additives are substances added to a water-soluble or readily available fertilizer that extend the time a particular form of nitrogen remains in the soil. In most cases, these products temporarily inhibit or delay but do not permanently prevent a specific process in the nitrogen cycle.

Nitrification Inhibition

Nitrification is the two-­step microbial process that converts ammonium (NH4 +) into nitrite (NO2­) and eventually into nitrate (NO3). The first step of the process is performed by specific ammonia-oxidizing bacteria and the second step by nitrite-­oxidizing bacteria.

Ammonium and nitrate are both plant-­available nitrogen forms, but ammonium is the more stable nitrogen form in the soil. Ammonium is a cation and can be retained by the soil’s cation exchange properties making it far less mobile than nitrate, an anion. Nitrate is susceptible to loss by runoff, leaching in permeable soils (sands and sandy loams) and denitrification (reduction to nitrous oxide (N2O) and dinitrogen (N2) gases) in poorly drained soils when anaerobic soil conditions persist. Applying ammonium or an ammonium­-forming nitrogen fertilizer is desirable since ammonium is less susceptible to denitrification and leaching losses. Eventually, the nitrification process converts ammonium supplied from fertilizer or mineralized from organic matter, manures or biosolids into nitrate.

Ammonia Volatilization Inhibition

Ammonia (NH3) volatilization is a soil surface loss mechanism that may occur when an ammonium or ammonium­-forming fertilizer is surface-applied and not incorporated immediately. All ammonium fertilizers can to some degree be lost via ammonia volatilization, but since the first reaction in the hydrolysis of urea is alkaline- forming, urea- containing fertilizers are more prone to loss by this mechanism than ammonium sulfate, which is an acid­-forming nitrogen fertilizer. Urea nitrogen when applied to a soil is hydrolyzed by the urease enzyme and converted first to ammonium carbonate and then to ammonia gas.

Urease is everywhere in the environment and can be found in soils and manures, as well as on plants and plant residues. The urease enzyme is believed to be released to the environment from living and disintegrating microbial cells. Losses of urea as ammonia gas increase as wind speed, soil moisture, humidity (>70%), soil pH and temperature increase. Under warm conditions, urea needs to be incorporated with tillage, rainfall (>0.5 inch rainfall) or irrigation within two days following application to significantly reduce gaseous ammonia loss.

Ammonia volatilization loss of surface-­applied urea from a dry silt loam soil can reach 20% to 30% in five days when the temperature exceeds 75° to ­80°F. The speed of the urea-urease reaction decreases as temperature declines, and thus there should be less ammonia volatilization loss of urea when applied in the winter to wheat compared to spring or summer applications to corn, cotton, grain sorghum or rice.

Ammonia volatilization loss of urea is much more rapid and extensive when urea is applied at high rates or to a muddy silt loam soil compared to a dry silt loam soil. Also, the ammonia loss potential tends to be lower in clay soils because of their higher cation exchange capacity compared to silt or sandy loam soils.

One approach to reduce potential ammonia volatilization loss of urea when immediate incorporation is not possible is to amend urea or urea­ ammonium­ nitrate with a chemical that inhibits or slows urease activity, or conversion of urea to ammonia gas. A urease inhibitor allows the prilled urea time to dissolve and move into the soil before hydrolysis to ammonia occurs, or allows time to be incorporated either mechanically or by water (e.g., by irrigation or rain.)